1. Field of the Invention
The invention relates generally to symbol timing recovery and more particularly to an apparatus and method for recovering a common symbol timing marker for multiple channel signals received with multiple antennas.
2. Description of the Prior Art
When a signal is transmitted over a channel with multiple propagation paths with different delays (multipath channels), the received signal is a linearly dispersed (smeared) version of the transmitted signal. Equalization and spatial diversity are used in existing systems for reducing the distorting effects of multipath. Spatial diversity requires multiple antennas for receiving a signal through multiple signal channels. Equalization uses an equalizer to recover the original transmitted signal from its dispersed version at the receiver, in the presence of additive noise.
The equalizer requires an estimate of the impulse response of the channel through which the transmission took place. In packet-based systems that require fast startup, the channel estimate is typically obtained by processing a known training sequence, such as a preamble, midamble or a synch word. The channel impulse response estimate is obtained as a series of samples or “coefficients” that represent the multipath delay spread profile of the channel during a particular packet.
Several equalizer structures exist in the art. A maximum likelihood sequence estimate (MLSE) equalizer provides optimum symbol detection. Unfortunately, the number of trellis states in an MLSE equalizer increases exponentially with the number of impulse response coefficients. Unless the packet time is accurately known and the delay spread is narrow, the complexity of the MLSE equalizer can be prohibitive.
Two other equalizer types commonly used today are the decision feedback equalization (DFE) equalizer and delayed decision feedback sequence estimation (DDFSE) equalizer. The DFE equalizer uses a feedforward filter and the DDFSE equalizer uses a Viterbi sequence estimator. However, the DFE and DDFSE equalizers are also prohibitively complex unless time is accurately known and delay spread is small because the number of taps in the feedforward filter and the number of states in the Viterbi sequence estimator increase with the number of coefficients of the channel impulse response.
Ariyavisitakul in U.S. Pat. No. 5,809,086 entitled “Intelligent Timing Recovery for a Broadband Adaptive Equalizer” incorporated herein by reference presents a technique for reducing the complexity of DFE or DDFSE equalizers. Unfortunately, the symbol timing recovery systems presented by Ariyavisitakul are operable only for a single signal channel. Anvari et al. in U.S. Pat. No. 6,130,909 shows a multichannel system having equalization. However, the system shown by Anvari et al. and all others so far as is known, would be expected to suffer from poor performance if any one of the multiple channels had a low signal-to-noise. There remains a need for a symbol timing recovery system for reducing equalizer complexity in a multichannel system.